The Women's Health Initiative is a randomized controlled clinical trial conducted in 40 clinical centers across the United States addressing some of the most common causes of death, disability, and impaired quality of life in postmenopausal women ages 50-79: cardiovascular disease; breast and colon cancer; and osteoporosis. The three components of this disease prevention trial are: (1) hormone replacement therapy with a primary outcome of coronary heart disease and stroke and secondary outcomes of hip and other fractures; (2) low fat dietary intervention with primary outcomes of breast cancer and colorectal cancer and a secondary outcome of coronary heart disease; and (3) calcium and vitamin D supplementation with a primary outcome of hip fractures and secondary outcomes of colorectal cancer and combined fractures. A concurrent observational study will provide estimates of health effects of known risk factors and new predictors for disease. The Clinical Coordinating Center develops the study protocol and manual of operations, as well as other study materials; trains clinical center staff on the execution of those procedures; collects, analyzes, and reports data; established and maintains central laboratories, a drug distribution center and central storage facility; arranges and coordinates committee meetings develops a data collection, management, analysis and reporting computer system; and maintain quality assurance and quality control procedures.